build(flake): add lua-language-server to devShell

Problem: lua-language-server is not available in the dev shell, making
it impossible to run local diagnostics checks.

Solution: add pkgs.lua-language-server to the devShell packages.

flake.nix

@@ -22,6 +22,7 @@
             pkgs.prettier
             pkgs.stylua
             pkgs.selene
+            pkgs.lua-language-server
           ];
         };
       });